1. Field of the Invention
The present invention relates to a Cu-based alloy and to a method of manufacturing a high strength and high thermal conductive forged article using the same.
2. Description of Related Art
Metallic materials having high strength and high thermal conductivity are used in members exposed to severe thermal fatigue, for example, thrust chambers of rocket engines, structures in fusion reactors (wherein one surface may contact a combustion gas of 3000° C. and the other surface may contact liquid hydrogen), and molds.
Examples of a high strength and high thermal conductive alloy used in the field include Cu-based alloy containing 0.8% (hereinafter all percentages are by weight in the present specification) of Cr and 0.2% of Zr as described in Japanese Unexamined Patent Application, First Publication No. Hei 4-198460. Generally, the Cu-based alloy is formed into a predetermined shape by forging and rolling after casting, and then the formed article is subjected to a predetermined heat treatment to obtain a high strength and high thermal conductive forged article. The tensile strength of the Cu-based alloy can be enhanced by controlling the conditions of a thermomechanical treatment while maintaining the thermal conductivity at a high level, regardless of it having the same composition.
However, since the service conditions of members of the apparatus became severe in view of the production of thermal stress and it was pointed out that a conventional material has a short lifetime up to the occurrence of cracking, higher thermal fatigue resistance has recently been required. To suppress the production of thermal strain of a metallic material, an improvement in thermal conductivity and an increase in thermal fatigue strength are required. Since the improvement in thermal conductivity has nearly reached the limit, it is desired to increase the thermal fatigue strength without reducing the thermal conductivity as compared with a conventional metallic material.
It has already been found that the tensile strength and the tensile proof stress are enhanced without reducing the thermal conductivity at a service temperature so as to enhance the thermal fatigue strength. To achieve the above object, there have been trials to increase the strength by further increasing a proportion of Cr or Zr in the above Cu-based alloy containing Cr (0.8%) and Zr (0.2%) as a base, thereby increasing a reduction ratio. When the proportion of Cr or Zr is increased and a fibrous fine structure is formed by swaging or wire drawing capable of introducing large strain in one direction, high strength can be obtained. However, contrary to expectations, the thermal fatigue strength is not increased because of poor ductility and sufficient forging and rolling cannot be conducted because of limits to the shape of the formed article, and thus it is difficult to obtain a desired strength in a formed article having any shape. Therefore, its application was limited to electrical members utilizing high strength and high electrical conductivity.
As described in Japanese Unexamined Patent Application, First Publication No. Hei 6-279894 and “Sakai et al., Journal of The Japan Institute of Metals, Vol. 55 (1991), pages 1382 to 1391”, a Cu-based alloy containing a large amount of Ag added therein has been developed as a novel alloy system. Similar to Cr or Zr, Ag has small solid solubility in Cu near room temperature and therefore exhibits a small decrease in thermal conductivity as a result of alloying. In the Cu-based alloy containing 8.5% or more of Ag added therein, a eutectic crystal is formed upon solidification. When an ingot of the Cu-based alloy, to which 15% of Ag was added to obtain a sufficient amount of a eutectic structure, is subjected to swaging or wire drawing during which large strain is introduced in one direction, like the above Cu—Cr—Zr alloy, the eutectic structure is broken to form a fiber-reinforced structure. Although the strength thus obtained is very high, it becomes necessary to conduct high reduction that enables a cast round bar to be formed into a wire rod having a diameter which is one-tenth that of the cast round bar, and thus a formed article having a certain measure or more of the wall thickness could not be obtained by this technique.